1. Field of the Invention
The present invention relates to SiC/2xxx Al composites, in particular to Al alloy composites in which ASTM 2000 series aluminum alloy are reinforced with SiC.
2. Description of the Background Art
Interfacial characteristics in metal matrix composites (MMCs) play an important role in determining the resultant composite properties. This is because superior material properties in MMCs are attributed to the load transfer from the matrix to the reinforcement through the interface. In general, interfaces in most MMCs consists of brittle intermetallic compounds, which sometimes can degrade the resultant composite properties. As a result, optimization of the interfacial characteristics in MMCs is meaningful not only to improve the material properties but also to control adequate process parameters required to obtain a desired interfacial strength.
In Al alloy composites reinforced with SiC particles (SiC.sub.p /Al alloy composites), a direct reaction between SiC and Al occurs to form hexagonal platelet-shaped Al.sub.4 C.sub.3 crystals and free Si. The interest in Al.sub.4 C.sub.3 and Si formed as a result of the interfacial reaction is stemmed from the fact that i) composites can be susceptible to some environments, such water, methanol, HCl, etc., due to the hydrophilic nature of Al.sub.4 C.sub.3 [1-4], ii) degradation of SiC itself occurs due to the formation of Al.sub.4 C.sub.3, which may cause decrease in strength and modulus, and iii) free Si, formed as a result of the interfacial reaction, produces Al--Si eutectic during fabrication or heat treatment stage [5], resulting in unintended mechanical properties of the matrix alloy. Therefore, fabrication of SiC.sub.p /Al alloy composites devoid of Al.sub.4 C.sub.3 has been one of the major concerns.
Among various methods which have been proven to be effective in achieving such a goal, there are two widely accepted methods are: i)addition of Si into the Al matrix [6-9], and ii) artificial oxidation of SiC to produce SiO.sub.2 layer on the surface of SiC [9-10]. A basic principle behind both methods is to enhance the Si activity, thereby reduce the Al activity, by dissolving a certain amount of Si within the Al matrix. Examples of the former method in the commercial application can be found from various melt-stir cast SiC.sub.p /Al composites produced by Duralcan which normally contain 9.5-10.5% of Si within the matrix. Detailed chemical composition and process temperatures of various Duralcan composites are shown in Table 1. However, when fabricating SiC.sub.p /wrought Al alloy composites, the use of the melt process may not be desirable due to the formation of interfacial reaction products. This is because most wrought Al alloys, such as 2xxx, 5xxx, 6xxx, 7xxx series Al alloys etc., normally contain less than 1% of Si so that the interfacial reaction cannot be avoided at temperatures where the matrix alloys exist as a liquid phase [5]. An example can be seen from the melt-stir cast SiC.sub.p -6061 Al composite made by Duralcan, as shown in FIG. 1.
TABLE 1 __________________________________________________________________________ Chemical composition of various Duralcan composites Temp Product* Si Fe Cu Mn Mg Ni Ti Zn Al (.degree. C.) Remark** __________________________________________________________________________ F3D.xxS 9.5- 0.8- 3.0- 0.5- 0.3- 1.0- 0.2 0.3 Rem 675- A380 10.5 1.2 3.5 0.8 0.5 1.5 max max 732 F3K.xxS 9.5- 0.2 2.8- -- 0.8- 1.0- 0.2 -- Rem 675- A339 10.5 max 3.2 1.2 1.5 max 732 F3N.xxS 9.5- 0.8- 0.2 0.5- 0.5- -- 0.2 0.3 Rem 675- A360 10.5 1.2 max 0.8 0.7 max max 732 F3S.xxS 8.5- 0.2 0.2 -- 0.45- -- 0.2 -- Rem 675- A359 9.5 max max 0.65 max 732 __________________________________________________________________________ *Duralcan F3D.xxS composites are general purpose diecasting composites, where F indicates Foundry, 3D corresponds to the matrix alloy, i.e., A380 Al Alloy in this case, and xxS denotes xx vol. % of SiC particle. **Commercial alloy systems similar to those of Duralcan composites.
Of interest here is that how much Si is required to prevent the interfacial reaction at various process temperatures. In recent years, various research works in an attempt to provide the answer for this question have been carried out both using experimental methods [5-8] and theoretical calculations [1, 5, 6, 11, 12]. The theoretical results, however, vary significantly depending on authors as shown in FIG. 2. This is because most researchers except Lloyd [1] and Lee [5] only considered the variation in the Si activity within the matrix, while the variations in activities of Al were not taken into account for the calculations. In addition, most studies carried out so far have considered the reaction between SiC and the molten Al only, not between SiC and Al in its solid or semi-solid state. Another drawback found in the previous studies was that the matrix of the composite was either the pure Al or the Al--Si binary alloy, not commercial alloys. Although the SiC.sub.p /Al composite system can be suitable for analyzing the relatively simple interfacial phenomena, results obtained from this system may have limitations in investigating detailed phenomena which could take place in commercial SiC.sub.p wrought Al composites.